Sodium carbonate regenerator

ABSTRACT

A sodium carbonate regenerator comprises a cyclone prechamber and a water-tube boiler. The cyclone prechamber is screened by tubes some of whose ends are secured in an inlet header, the other ends being fixed in an outlet header. A water-tube boiler drum communicates with the outlet header by means of riser tubes and with the inlet header via a heat exchanger by means of downtake tubes.

' United States atent Oni et al.

[ SODIUM CARBONATE REGENERATOR [76] Inventors: Lazar Adolfovich Oni, prospekt Kosmonavtov l9, korpus 4, kv. 35; Svetlana Sergeevna Timofeeva-Tomskaya, Novocherkassky prospekt, 29, kv. 1; Jury Nlkolaevich Korchunov, ulitsa Orbeli 23, kv. 45; Viktor Yalsovlevich Levi, Ligovsky prospekt 87, kv. 48, all of Leningrad; Nikolai Lvovich Borisov, Kotlozavodskaya ulitsa 12g, kv. 4, Belgorod; Nikolai Vasilievich Reprev, ulitsa B. Khmelnitskogo 124, kv. 68, Belgorod; Anna Fedorovna Leontievskaya, ulitsa B. Khmelnitskogo 124, kv. 5, Belgorod; Viktor lvanovich Ryabokobylenko, ulitsa Kotlozavodskaya 8, kv. 6, Belgorod; Nikolai Stefanovich Voronov, ulitsa Kotlozavodskaya 8, kv. 7, Belgorod, all of USSR.

[22] Filed: Apr. 10, 1973 [21] Appl. No.: 349,812

[52] US. Cl. 23/262; 23/277 R; 122/7 C; l22/DlG. 2; 423/207 Oct. 28, 1975 [51] Int. Cl. 1301,] 6/00; COlD 7/07 [58] Field of Search 23/262, 277 R; 423/207. 423/428, 566; 122/D1G. 2, 7 C, 360

[56] References Cited UNITED STATES PATENTS 2,673,787 3/1954 Greenawalt 23/262 X 2,681,047 6/1954 Dalin et a1. l22/DIG. 2 2,703,073 3/1955 Klein et al. 122/360 2,739,878 3/1956 Jolley 23/262 X 3,213,831 10/1965 l-lochmuth 122/7 C X 3,867,251 2/1975 Holme 423/207 X Primary Examiner-Morris O. Wolk Assistant Examiner-Barry 1. Hollander Attorney, Agent, or Firm-Waters, Schwartz & Nissen 5 7 ABSTRACT A sodium carbonate regenerator comprises a cyclone prechamber and a water-tube boiler. The cyclone prechamber is screened by tubes some of whose ends are secured in an inlet header, the other ends being fixed in an outlet header. A water-tube boiler drum communicates with the outlet header by means of riser tubes and with the inlet header via a heat exchanger by means of downtake tubes.

1 Claim, 1 Drawing Figure US. Patent Oct. 28, 1975 SODIUM CARBONATE REGENERATOR A sodium carbonate regenerator comprises a cyclone prechamber and a water-tube boiler. The cyclone prechamber is screened by wall tubes some of whose ends are secured in an inlet header and the other ends are fixed in an outlet header. A water-tube boiler drum communicates with the outlet header by means of uptake tubes and with the inlet header by means of downtake tubes via a heat exchanger.

The present invention relates to machinery used in the paper and pulp industry for the regeneration of chemicals and the generation of power steam through the combustion of liquid waste of pulp production and more particularly to sodium carbonate regenerators.

The present invention is practical when used for burning up a black-ash liquor in the sulfate production of pulp.

The sulfate pulp production involves pulping in a boiling solution which is a water solution of active components of sodium hydroxide (NaOH) and sodium sulfide (Na S) at elevated temperature and pressure with a subsequent water washing. The washing results in producing the pulp and the water solution of a blackash liquor, the latter being the active components of the boiling solution, chemically bound with lignin.

The active components of the boiling solution make up a mineral portion of the black-ash liquor.

Apart from the bound mineral compounds, the black-ash liquor also contains free salts: sodium hydroxide (NaOH), sodium sulfide (Na-2S), sodium sulfate (Na SO and other admixtures in minor quantities.

The known process of pulp production and the process of combustion of the black-ash liquor are accompanied by the loss of sodium salts and sulfur. This loss is made up for by adding sodium sulfate to the blackash liquor prior to the combustion of the latter. During the combustion sodium sulfate is reduced to become an active component of the boiling solution, i.e., sodium sulfide, owing to the reaction of sodium sulfate with the carbon of the organic portion of the liquor, according to the formula:

Lignin as well as resin substances of wood pulp and sugar make up the organic portion of the black-ash liquor.

The combustion of the black-ash liquor helps resolve two problems:

first, the separation of the mineral portion in the form of sodium carbonate and sodium sulfide (Na CO and Na S) and return of the newly obtained salts into a pulp production cycle after their causticizing;

second, the utilization of the heat resulting from the combustion of the organic portion, accounting for some 60 percent of the dry mass of the black-ash liquor, permits to generate steam of power parameters, the amount of the generated steam being largely in compliance with the requirements of the pulp production process, and the generated electric power exceeding the requirements of production to an extent that the greater the power, the higher are the parameters of the furnace chamber completely screened by tubes, steam superheating and economizer heating surfaces. The furnace chamber is limited by a hearth made of wall tubes in its lower portion and by a steam superheater surface in its upper portion. The height of the furnace chamber in sodium carbonate regeneration units with a capacity of 1400 tons of the burned dry mass of the black-ash liquor per day (steam production capacity equal to 200 t/h) is 2225 m.

Provided on the side walls of the furnace chamber, some 5 m high from the hearth, are coarse'spray injectors through which the black-ash liquor is fed to the furnace.

Arranged in two or three horizontal sections throughout the height of the furnace chamber are blast nozzles. The blast nozzles of the lower floor are spaced uniformly along the perimeter of said chamber at a height of some 1.2 m from the hearth. In case the furnace chamber has three floors of blast nozzles, the second floor of said blast nozzles is provided below the injectors that feed the black-ash liquor.

The blast nozzles of the second floor are also spaced uniformly along the perimeter of the furnace chamber.

The upper floor of the blast nozzles is 2.5-3.0 m higher than the section accommodating the liquorfeeding injectors and is made as largesize nozzles positioned in the corners of the furnace chamber so that the direction of an air flow escaping therefrom is tangential to a circumference with a diameter of 1.5 2.0 m, whose center is on a vertical axis of the furnace chamber.

Cooling of furnace gases formed as a result of the combustion of the black-ash liquor is carried out due to the heat exchange in the furnace chamber, in the steam superheating surface and in the economizer heating surface.

A detailed description of the design of a modern sodium carbonate regenerator is furnished in an article titled Present-day sodium carbonate regenerators by Ye. l. Dorman and S. S. Timofeyeva-Tomskaya, published in the Bumazhnaya promyshlennost (Paper Industry) magazine, Nos 8 and 9, 1964, Moscow.

The organization of the processes of combustion of the black-ash liquor and regeneration of chemicals in the above-described sodium carbonate regenerator is characterized by the following.

The black-Ash liquor with a moisture content of 3545 percent is fed to the furnace chamber through the coarse-spray injectors. The liquor drops fall toward the hearth of the furnace chamber, reacting with a flow of the liquor combustion products heated to l,l00l,200C. Ash is deposited onto the hearth of the furnace chamber to form a tapering ash zone thereon.

Like the mineral portion of the black-ash liquor, the ash also contains a carbon as a product of mechanical underfiring.

Fine liquor drops have time to burn up completely within the furnace chamber space, while the remaining mineral portion in the form of the finely dispersed particles of sodium sulfate and sodium carbonate are carried into the boiler gas flues.

As a result of the reaction of the carbon of mechanical underfiring with sodium sulfate, the latter is reduced to sodium sulfide in the ash zone on the furnace chamber hearth. This process takes place in a reducing atmosphere at l,000-1,100C with a number of side reactions separating hydrogen sulfide. vapours of elementary sulfur, etc.

To create a reducing atmosphere in the lower portion of the furnace chamber, some 35 to 60 percent of the totally consumed air should be supplied through the lower floor of the blast nozzles. The rest of the air is fed directly to the furnace chamber and utilized for an allout combustion of the organic portion of the black-ash liquor.

During the combustion of the organic portion, sodium hydroxide (NaOI-I) bound with it reacts with carbon dioxide (CO to form sodium carbonate (Na CO i.e. soda.

The melt of sodium carbonate and sodium sulfide produced as a result of the reduction of sodium sulfate is collected on the furnace chamber hearth to be then discharged therefrom for further treatment to prepare a salt solution good for pulping. With air in short supply, the sodium sulfate reduction achieved in the lower portion of the furnace chamber goes on with side reactions separating hydrogen sulfide and vapours of elementary sulfur and is responsible for a number of disadvantages typical of sodium carbonate regenerators.

Research into high-temperature corrosion of wall tubes and experience in operating sodium carbonate regenerators proved that in the lower portion of the furnace chamber, with its high concentration of gaseous and mineral compounds that are aggressive to metal, an increase in the temperature of the metal tubes to 350C and higher causes intensive corrosion and even flaws.

In the upper portion of the furnace chamber, the concentration of compounds that leads to the metal corrosion tends to decrease. Yet-the temperature of the steam after the steam superheater higher than 480C provokes a sharp stepping-up of the metal corrosion.

For reasons specified hereinabove, the dependable operation of sodium carbonate regenerators is limited by 64 technical atmospheres of the boiler drum pressure and by 480C of the superheated steam. A technical atmosphere, usually designated at, the unit of pressure, is defined in the metric technical system, equal to l kilogram-force per square centimeter. It might be added for the sake of completeness that the designation ata is a similar unit of absolute pressure in the same metric technical system, which is equal to one technical atmosphere. Limitations on the parameters of the steam being generated affects the operating economy of a power station at a paper and pulp factory housing sodium carbonate regenerators.

In cases when the units are nevertheless operated at elevated steam pressure and temperatures, special and expensive measures have to be adopted to protect the tubes against corrosion and to detect it in time. These are: provision of bimetallic wall tubes, regular inspection of these tubes with measuring wall thickness, and more frequent, compared with power boilers, replacement of screening surfaces. Sodium carbonate regenerators operated at a drum steam pressure of below 64 technical atmospheres are not free from hightemperature corrosion as the deposition of iron, phosphates and hardness salts in the walls from the side washed by the boiler water causes a drop-in the heat transfer coefficient and a rise in the temperature of the wall tubes. In many instances the tube temperature reached danger values and flaws emerged due to hightemperature corrosion.

The danger resultant fromthe emergence of flaws due to the high-temperature corrosion of the wall tubes is aggravated by the massing of large quantities of the molten sodium salts on the furnace chamber hearth. The reaction of the boiler water with the melt of these salts causes devastating blasts. Thus, for instance, in the USA, more than 15 percent of all the mounted units were destroyed by furnace explosions.

Damage produced by such troubles exceeds, in many cases, the cost of repair as it causes protracted standstill of the entire paper and pulp production or a part of it.

On the other hand, the organization of the furnace process with the distribution of air required for combustion in two or three horizontal sections of the furnace chamber is a cause of an insufficiently effective mixing of the fuel with air. This is explained by the fact that the bulk of the air supplied through the lower floor of the nozzles loses energy in proximity to the wall tubes upon hitting against the ash zone and climbs along the screens as an inert, deprived of speed in flow.

The unfavourable aerodynamic organization of the furnace process leads to an increased height of the furnace chamber, which is required for an adequately complete combustion of the products of chemical underfiring, hydrogen sulfide and other sulfur compounds with bad odour. As a result, the construction depth, metal consumption and, naturally, cost of sodium carbonate regenerators exceeds by far the respective figures typical of the construction of power units of similar parameters and steam production. For instance, the specific consumption of metal per ton of steam production is 10-12 tons per ton for sodium carbonate regeneration units, while it is 56 tons per ton for power boilers. A similar difference can be found when comparing the costs of such units. Note should be made that despite an increased height of the furnace, it is practically impossible to avoid the ejection of considerable amounts of bad-smelling, sulfur containing compounds into the atmosphere. This circumstance, combined with steadily increasing sanitary requirements to the clearness of air, compells industries to plan considerable expenditures on the construction of gas-purifying installations.

Also known are the power and power-process boiler units with a furnace made as a vertical or horizontal cyclone prechamber.

Specifically, power-process boiler units are known in the art, intended for melting and reprocessing mineral raw materials, or for utilizing production refuse containing organic or mineral compounds.

Thus, for instance, cyclone prechambers are effective in the pulp industry for burning up sulfite liquors, including those that are sodium-based.

The use of cyclone prechambers is increasing for the continuous production of sodium sulfide from sodium sulfate.

A boiler unit comprises a vertical cyclone prechamber screened by walls and a water-tube boiler with steam superheating and economizer surfaces.

The upper portion of the cyclone prechamber accommodates blast nozzles with devices designed to feed a fuel to be burned mounted therein, e.g., fuel injectors.

Arranged beneath the cyclone prechamber is a melt collector screened by the tubes and being, at a time, a gas flue communicating the cyclone prechamber with the water-tube boiler. Provided between the gas flue and the boiler is a cinder-catching bundle.

Some of the ends of the wall tubes of the cyclone prechamber are secured in an inlet header and the other ends in the outlet header. The outlet header is connected with a water-tube boiler drum by means of uptake tubes, the inlet header communicating with said water-tube boiler drum by means of downtake tubes. The wall tubes that screen the cyclone prechamber and the melt collector form, together with the uptake and downtake tubes connected with the boiler drum, an independent boiler water circulation loop.

The evaporation surfaces of the cinder-catching bundle and of the water-tube boiler are connected with the inlet and outlet headers by means of the uptake and downtake tubes to form a number of independent boiler water circulation loops.

The combustion of the black-ash liquor in the sodium carbonate regenerator with a cyclone prechamber, and the organization of the reduction of sodium sulfate in the film of the melt running it off, permits to eliminate a number of disadvantages in the construction of modern sodium carbonate regenerators, specifically, to reduce the sizes of a regeneration unit by eliminating a high furnace chamber whose wall tubes operate at low heat loads.

Thanks to improved aerodynamic organization of the process, the complete combustion of the black-ash liquor is ensured within the cyclone prechamber. This explains the absence of a reducing atmosphere behind the cinder-catching bundle, along with the absence of attendant gaseous sulfur-containing compounds and mineral salts, i.e., hydrogen sulfide, sulfur vapours and sodium sulfide, aggressive to the metal of the wall tubes.

The combustion products behind the cinder-catching bundle consist of fully oxided gaseous compounds, e.g., C0 H 0 and S0 while the mineral portion forced out of the cyclone prechamber includes sodium sulfate and sodium carbonate, which helped eliminate the danger of an intensive corrosion of the wall tubes of the boiler and the steam superheating surface. The corrosion of the wall tubes and the steam superheating surface of the water-tube boiler will not exceed a level typical of fuel-oil burning power boilers. Still, problems arising due to the high-temperature corrosion in the metal of the wall tubes of the very prechamber remain and as before restrict an increase in the parameters of a generated steam without affecting the dependable operation of the unit.

It is an object of the present invention to increase the dependability and service life of the sodium carbonate regeneration units with a simultaneous reduction in their production and maintenance costs.

One more object of the invention is to increase the operating economy of a power station by increasing the parameters of a generated steam without affecting the dependable operation of a sodium carbonate regenerator.

The primary object of the invention is to provide a sodium carbonate regenerator with its dependable operation during the combustion of a black-ash liquor, the pressure in a water-tube boiler drum being above 64 technical atmospheres and the temperature of superheated steam higher than 480C.

According to the abovestated and other objects, the essence of the present invention is that in a sodium carbonate regenerator comprising a cyclone prechamber screened by wall tubes, some of whose ends are secured in an inlet header and the other ends in an outlet header, and a water-tube boiler whose drum communicates with said outlet header by means of uptake tubes, according to the invention, said inlet header communicates with the above drum by means of down-take tubes via a heat exchanger.

It is suggested that the heat exchanger be made airto-water.

It is also recommended that from its air side said heat exchanger be connected with the cyclone prechamber.

The thus embodied sodium carbonate regenerator is by far more dependable and has a longer service life thanks to the provision of the air-to-water heat exchanger prior to the entry of the boiler water to the wall tubes of the cyclone prechamber. This permits controlling the temperature of the boiler water at the inlet of the wall tubes and maintain it below a saturation temperature corresponding to the pressure in the boiler drum.

Thus, for instance, the drum pressure being 1 10 technical atmospheres and the saturation temperature that corresponds to this pressure being 316C, the temperature of the wall of a tube may reach 360C and higher from the water side, considering thermal resistance. When the boiler water passes through the heat exchanger into the wall tubes of the cyclone prechamber, the water temperature drops to 220250C, which is absolutely safe as far as the emergence of corrosion is concerned, even in case of depositions in the wall tubes from the side of the boiler water circulating therein. Elevation of the parameters of the produced steam without affecting the dependability of the unit increases the operating economy of a power station. Parallel to water cooling, heating of air takes place, which are fed to the furnace space of the cyclone prechamber and used for the combustion of the black-ash liquor.

Other peculiar features and advantages of the invention will be more apparent from the following description of its exemplary embodiment and the sole FIG- URE of the accompanying drawing which shows schematically a sodium carbonate regeneration unit according to this invention.

The sodium carbonate regenerator is made up of a cyclone prechamber l and a water-tube boiler 2. The cyclone prechamber 1 is screened by wall tubes 3 some of whose ends are secured in an inlet header 4 and the other ends are secured in an outlet header 5. The wall tubes 3 of the cyclone prechamber 1 form a melt collector 6 in its lower portion. The outlet header 5 of the cyclone prechamber 1 is connected with a drum 7 of the watertube boiler 2 by means of uptake tubes 8.

The inlet header 4 communicates with the drum 7 by means of downtake tubes 9 via an air-to-water heat exchanger 10. Provided between said air-to-water heat exchanger 10 and the drum 7 is a circulating pump 11. The upper portion of the cyclone prechamber l accommodates blast nozzles 12 so that the outcoming air stream is directed tangentially to the inner surface of the cyclone prechamber 1. Arranged inside the blast nozzles 12 are injectors 13. The water-tube boiler 2 is screened by tubes 14. The cyclone prechamber 1 communicates with the water-tube boiler 2 through a gas flue 15 also screened by tubes. Said gas flue l5 incorporates a tube cinder-catching bundle 16 with an inlet header 17 and an outlet header 18.

The water-tube boiler 2 accommodates evaporation surfaces 19, steam superheating surfaces 20 and economizer surfaces 21.

The inlet header 4 of the cyclone prechamber l and an inlet header 22 of the economizer surface 21 communicate with a pipe 23 supplying a feed water. The connection of the outlet header 4 of the cyclone prechamber 1 with the pipe 23 is effected via a pipe 24 and a controller 25.

Air required for burning up the black ash liquor is taken by a fan 26 through a suction duct 27 from the boiler room. The air-to-water heat exchanger has connection with the blast nozzles 12 of the cyclone prechamber 1 by means of an air blower 28.

The sodium carbonate regenerator of the invention operates as follows. A black ash liquor with a moisture of 3545 percent, preheated to 115120C, is fed to the cyclone prechamber 1 via the injectors 13.

Also supplied thereto is air through blast nozzles 12 that communicate, via the air blower 28, with the air portion of the heat exchanger 10, said air portion being connected with a fan 26 taking air from the boiler room through the air suction duct 27. The combustion of the black ash liquor results in melting its mineral portion, reducing sodium sulfate to sodium sulfide and generating furnace gases.

As this takes place, the reduction of sodium sulfate to sodium sulfide is possible owing to the reaction of the melted sodium sulfate with the products of chemical and mechanical underfiring. The obtained melt of salts consisting essentially of sodium carbonate (Na CO and sodium sulfide (Na S) is collected in the melt collector 6 wherefrom it is discharged for further use.

Finely dispersed particles of the salt melt are carried by the furnace gases into the gas flue where they are deposited on the cinder-catching bundle l6 and then return into the melt collector 6. The total degree of catching the mineral portion of the black ash liquor in the cyclone prechamber 1 and the cinder-catching bundle 15 reaches 90-95 percent. Through the gas flue 15, the furnace gases go to the water-tube boiler 2 cooling off therein through contacting with the evaporation surfaces 19, steam overheating surfaces and economizer surfaces 21. The steam-and-water mixture obtained in the evaporation surfaces 19 is fed to the drum 7 wherein steam is separated from water. The steam comes to the steam overheating surfaces 20, overheated to 510-530C and then supplied to the consumer.

The wall tubes 3 of the cyclone prechamber 1 and the melt collector 6, the wall tubes (not shown in the figure) of the gas flue l5 and the tubes of the cindercatching bundle 16 are cooled with boiler water that comes to the input header 4 of the cyclone prechamber 1 and to the cinder-catching bundle 16. The boiler water shown with pointer in the figure, that cools off the above wall tubes, is fed through the downtake tube 9 to the circulating pump 1 1 from the drum 7. By said pump 11 the boiler water is supplied to the air-to-water heat exchanger 10, cooled off therein and further fed to the inlet header 4 of the cyclone prechamber l and to the inlet header 17 of the cindercatching bundle 16. From the inlet header 4 the boiler water goes to the wall tubes 3 of the cyclone prechamber l to be heated therein. From the inlet header 17 of the cindercatching bundle 16 the boiler water rises, by tubes, to the outlet header 18. From the wall tubes 3 the heated boiler water is collected in the outlet header 5 wherefrom it is taken to the drum 7 of the water-tube boiler 2 via the uptake tube 8.

The consumption of the boiler water through the airto-water heat exchanger 10 is selected such that the temperature of the metal of the wall tubes 3 of the cyclone prechamber 1 and the tubes of the cindercatching bundle 16 should not exceed the values that might cause high-temperature corrosion at any place of their contact with the melt and the products of the combustion of the black-ash liquor, that feature high concentration of compounds (sodium sulfide, sulfur vapours, hydrogen sulfide) aggressive to metals.

The feed water is fed through the pipe 23 to the inlet header 22 of the economizer surface 21 and to the inlet header 4 of the cyclone prechamber l and the inlet header 17 of the cinder-catching bundle 16 through the pipe 24 via the controller 25.

On passing through the economizer surface 21 and the wall tubes 3 of the cyclone prechamber l, the feed water gets inside the drum 7 of the water-tube boiler 2.

The consumption of the feed water through the pipe 24 is adjusted so that the temperature of the metal of the wall tubes 3 of the cyclone prechamber l and the tubes of the cinder-catching bundle 16 should not ex ceed the values that might cause high-temperature corrosion.

Naturally, those skilled in the art may introduce various alterations and modifications into the proposed sodium carbonate regenerator described substantially with reference to an exemplary embodiment thereof, said alterations and modifications considered as falling within the essence and scope of the invention.

What we claim is:

l. A sodium-carbonate regenerator for the combustion of black-ash liquor by the aid of air, with the simultaneous regeneration of agents, including the reduction of sodium sulfate carried out in a film of the melt processed therein, comprising, in combination: a cyclone prechamber for burning the liquor and reducing the sodium sulfate, having a plurality of substantially vertically secured metallic screening wall tubes therein, the melt being made to flow down along said wall tubes, which latter are respectively closed by bottom inlet and top outlet headers; a water-tube boiler communicating with said prechamber through a gas flue; said boiler serving to generate steam having a pressure of approx. technical atmospheres, and in the superheated condition a temperature of from 510 to 530C; a drum to which lead uptake tubes and from which lead gravitational downtake tubes, the former communicating with said outlet header while the latter communicate with said inlet header; an air-to-water heat exchanger having an air portion and a water portion, and being installed in said downtake tubes; said wall tubes, said outlet header, said uptake tubes, said drum and said downtake tubes, as well as said water portion of the heat exchanger being serially connected in an independent circulation loop; blast nozzle means leading from said air portion of heat exchanger to at least one upper portion of said prechamber in the area of said outlet headers, to assist the combustion; means for providing forced circulation of the boiler water through said loop; and means for cooling the boiler water from said heat exchanger and feeding the same by way of said loop in the cooled condition to said inlet header. 

1. A SODIUM-CARBONATE REGENERATOR FOR THE COMBUSTION OF BLACK-ASH LIQUOR BY THE AID OF AIR, WITH THE SIMULTANEOUS REGENERATION OF AGENTS, INCLUDING THE REDUCTION OF SODIUM SULFATE CARRIED OUT IN A FILM OF THE MELT PROCESSED THEREIN, COMPRISING, IN COMBINATION: A CYCLONE PRECHAMBER OF BURNING THE LIQUOR AND REDUCING THE SODIUM SULFATE, HAVING A PLURALITY OF SUBSTANTIALLY VERTICALLY SECURED METALLIC SCREENING WALL TUBES THEREIN, THE MELT BEING MADE TO FLOW DOWN ALONG SAID WALL TUBES, WHICH LATTER ARE RESPECTIVELY CLOSED BY BOTTOM INLET AND TOP OUTLET HEADERS, A WATER-TUBE BOILER COMMUNICATING WITH SAID PRECHAMBER THROUGH A GAS FLUE, SAID BOILER SERVING TO GENERATE STEAM HAVING A PRESSURE OF APPROX. 110 TECHNICAL ATMOAPHERES, AND IN THE SUPERHEATED CONDITION A TEMPERATURE OF FROM 510* TO 530*C, A DRUM TO WHICH LEAD UPTAKE TUBES AND FROM WHICH LEAD GRAVITATIONAL DOWNTAKE TUBES, THE FORMER COMMUNICATING WITH SAID OUTLET HEADER WHILE THE LATTER COMMUNICATE WITH SAID INLET HEADER, AN AIR-TO-WATER HEAT EXCHANGER HAVING AN AIR PORTION AND A WATER PORTION, AND BEING INSTALLED IN SAID DOWNTAKE TUBES, SAID OUTLET HEADER, SAID UPTAKE TUBES, SAID DRUM AND SAID DOWNTAKE TUBES, AS WELL AS SAID WATER PORTION OF THE HEAT EXCHANGER BEING SERIALLY CONNECTED IN AN INDEPENDENT CIRCULATION LOOP, BLAST NOZZLE MEANS LEADING FROM SAID AIR PORTION OF HEAT EXCHANGER TO AT LEAST ONE UPPER PORTION OF SAID PRECHAMBER IN THE AREA OF SAID OUTLET HEADERS, TO ASSIST THE COMBUSTION, MEANS FOR PROVIDING FORCED CIRCULATION OF THE BOILER WATER THROUGH SAID LOOP, AND MEANS FOR COOLING THE BOILER WATER FROM SAID HEAT EXCHANGER AND FEEDING THE SAME BY WAY OF SAID LOOP IN THE COOLED CONDITION TO SAID INLET HEADER. 